Piston head construction



Aug. 19, 1958 A. D. MGDUFFIE 2,343,287

PISTON HEAD CONSTRUCTION Filed June 14, 1955 Am\\\\\\\\\m m, n 12\ \3 x O O INVENT OR ATTORNEY United States Patent PISTON HEAD CONSTRUCTION Archie D. McDufiie, Berkley, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application June 14, 1955, Serial No. 515,293

4 Claims. (Cl. 309-11) The present invention relates to the construction of a piston head and more particularly a construction in which thermal distortion of the piston is considerably reduced or eliminated.

Most present-day internal combustion engines utilize pistons in which the head or top of the piston is considerably thicker or more massive than the skirt. A specific variation of this relationship between head and skirt mass is the Wedge-shaped piston head with respect to which the instant invention has been illustrated. It is to be understood that the invention is not limited to this wedge-type piston head construction, but rather is generally applicable to pistons having thickened heads.

It has been found that wedge-shaped piston heads have definite operational advantages numbered among which is the reduction of detonation and attendant attainment of higher compression ratios. In making such a piston, however, a considerable mass of material is concentrated at the top of the piston and which mass is subjected to the high heat of the combustion chamber. Accordingly, the large mass of material in the piston head in expanding and contracting under changes in operating temperatures within the combustion chamber tends to distort the piston, more particularly the piston skirt, thus resulting in inefiicient operation of the piston due mainly to unequal wear within the piston cylinder. It is with the purpose of avoiding this kind of piston distortion that the present piston construction has been developed.

Essentially, the present invention seeks to avoid piston distortion by providing a series of expansion joints within the piston head. It has been found in the course of the development of such a non-distorting type piston that not only is the concept of such expansion joints important but also the type and particular location of these expansion joints is significant.

It has also been found that care must be taken in providing an expansion joint of such a size that it does not cause or facilitate detonation. Finther, the expansion joint or joints must be so oriented with respect to the center of the piston and heat concentration as not to interfere with the transfer of heat from this center to the cooler piston belt area. Care must also be taken with respect to the size, number and positioning of the expansion joints not to structurally weaken the piston head construction and thereby reduce its gas load carrying ability.

It iswith these purposes in mind that the present invention has been developed and will hereinafter be set forth in greater detail.

In the drawings:

Figure 1 is an elevational view of a piston head embodying a plurality of expansion joints.

Figure 2 is a sectional view along lines 2-2 of Figure 1 taken through one of the expansion joints.

Figure 3 is a plan view of the piston head showing the general orientation of the expansion joints.

Referring to the drawings, a piston is shown generally at 11 and it includes a head portion 12 and a skirt portion 2,848,287 Patented Aug. 19, 1958 'ice ficient of expansion, the appreciable differential in temperature between the piston head and the skirt causes theformer to expand at a greater rate than the latter thus distorting the piston and causing uneven wear.

As illustrated, particularly in Figure 2, head 12 includes a crescent shaped wedge portion 14 which in this particular instance is of a double-stepped cross-section to provide desired operating characteristics. The head is also provided with the conventional annular grooves 16 adapted to receive the piston rings. Depending from the center portion of the head 12 is a Webbed supporting structure 17 for a wrist pin bearing 18. As best seen in Figure 2 the wrist pin bearing axis is substantially normal to any longitudinal wedge section.

Laterally extending from the wrist pin bearing supporting structure are flange members 19 having piston skirt sections 21 supported on or formed integrally at the outer end thereof. As best seen in Figure 2, sections 21 cooperate with the piston head 12 to form a partially cylindrical piston trunk.

Referring to Figs. 1 and 3, it' will be seen that expansion grooves 22 are formed in the piston head 12. It has been found, particularly with wedge-shaped piston heads as shown, that the thermal expansion of the piston head is predominantly in the direction of the Wrist pin axis. Accordingly, for the expansion grooves to be most effective in permitting non-distortion type expansion, the grooves should be disposed transversely to the wrist pin axis 23 so as to dissect the wedge. With the grooves thus located, it is apparent that as the piston head tends to expand in the direction of the wrist pin axis the expansion will be into the appropriate expansion grooves thus reducing any tendency to distort the remainder of the piston.

The invention is not limited to placing the expansion grooves 22 in a generally perpendicular relationship with the wrist pin axis 23. If other factors so dictate, it is possible for the grooves to be inclined to the aforesaid axis with the understanding, however, that as the angularity of the grooves proceeds from the position, as shown, to a 0 or parallel position with respect to the wrist pin axis, the grooves become correspondingly less effective in accommodating the head expansion without piston distortion.

While not shown in the drawings, it is fully comprehended that similar expansion grooves may be formed in the underside of the piston head as a means of further reducing any tendency of piston distortion.

It is to be noted that the expansion grooves 22 are substantially linear in form and proceed generally from the center of the piston outwardly toward the piston belt area. Inasmuch as the heat concentration is the greatest at the center of the piston head, it is desirable to have the head constructed so that heat may be transferred from the center of the piston to the cooler outer periphery thereof in relatively straight lines. While they might function commendably to permit head expansion, it is apparent that arcuate or curvilinear type grooves would reduce the ability of the piston head to transfer heat to the periphery of the piston head as contrasted with the linear section shown. In other Words, a potentially undesirable characteristic of the expansion grooves 22 is that they individually constitute insulating devices and as such it is imperative that they be disposed so as to provide minimal impedance to the straight line flow of heat from the center of the piston to the belt area.

While the exact number of expansion grooves appears to be relatively unimportant to the successful operation of the subject invention, it is definitely advisable not to employ too many grooves lest the structural strength of the piston head itself be significantly weakened. It has been found that three grooves, as shown, represent a number and grouping of expansion joints which has substantially eliminated piston distortion in the type of piston shown. It is obvious that it may be necessary to change the number and grouping of the expansion joints with variously shaped piston heads.

It is important also to realize that a single groove having a total cross sectional area equal to that of a plurality of narrower grooves is not deemed to be the equiv alent of using a plurality of such grooves. The most important reason for this-lack of equivalency relates to the matter of detonation. In other words, when using such expansion grooves it is anticipated that they will eventually fill up with carbon particles which may cause secondary burning or detonation. With very thin or narrow expansion grooves, however, the individual quantities of carbon occur in small enough amounts not to present a serious potential source of secondary ignition. A single large expansion groove, on the other hand, would contain a relatively large quantity of carbon which would be very likely to cause detonation within the combustion chamber.

The general limitation on the width or narrowness of the expansion slot or grooves appears to be as. small as can be practically made with tools available for such It is definitely true that the nar-.

section, and expansion grooves formed in said head permitting expansion of said head in the direction of the wrist pin axis without resultant distortion of the remainder of the piston.

2. A non-distorting piston for an internal combustion engine comprising a head, a skirit and a wrist pin bearing, said head including a crescent-shaped wedge portion projecting upwardly therefrom, the wrist pin bearing axis being substantially normal to any longitudinal wedge section, and a plurality of upwardly opening parallel grooves dissecting said wedge, said grooves being disposed substantially normal to the axis of said wrist pin bearing and extending generally from the center of the piston head toward the periphery thereof;

3. A non-distorting type piston for an internal combustion engine, said piston having a wedge-shaped head portion, a wrist pin bearing, the bearing axis being substantially normal to any longitudinal wedge, section, said wedge portion including a plurality of parallel related narrow grooves formed therein, said grooves being generally normally disposed to the bearing axis to permit non=piston distorting expansion of said head.

4. A piston. as set forth in claim 3 in which each of said grooves extends generally from the center of the piston head to the periphery thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,320,064 Junkers Oct. 23, 1919 1,527,993 Norton Mar. 3, 1925 2,100,143 Mock Nov. 23, 1937 FOREIGN PATENTS 557,241 France Apr. 30, 1923 325,194 Great Britain Feb. 3, 1930 125,324 Australia May 23, 1944 826,994 Germany Ian. 7, 1952 

